The present invention relates to shielding gaskets, and more specifically to a strip gasket utilizing a low closure force on an enclosing face to provide electromagnetic interference (EMI) shielding or radio frequency interference (RFI) shielding.
The operation of electronic equipment, such as televisions, radios, computers, medical instruments, business machines, communication equipments, and the like, is typically accompanied by the generation of radio frequency and/or electromagnetic radiation within electronic circuitries of an electronic system. The increasing operating frequency in commercial electronic enclosures, such as doors and access panels, housings for shielding computer cabinets and drives, cathode ray tubes (CRTs) and automotive electronic modules, results in an elevated level of high frequency electromagnetic interference (EMI). Any gap between the metal surface confronting or mating with the doors and access panels affords an opportunity for the passage of electromagnetic radiation and the creation of electromagnetic interference (EMI). These gaps also interfere with the electric currents running along the surfaces of the cabinets from EMI energy, which is absorbed and is conducted to the ground.
If not properly shielded, such radiation can cause considerable interference with unrelated equipment. Accordingly, it is necessary to effectively shield and ground all sources of radio frequency and electromagnetic radiation within the electronic system. Therefore, it is advisable to use a conducting shield or gasket between such surfaces to block the passage of the electromagnetic interference (EMI).
To attenuate EMI effects, shielding gaskets having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device, and to insulate that device or other “target” devices from other source devices. Such shielding is provided as a barrier which is inserted between the source and the other devices, and is typically configured as an electrically conductive and grounded housing which encloses the device. As the circuitry of the device generally must remain accessible for servicing or the like, most housings are provided with openable or removable accesses such as doors, hatches, panels, or covers. Between even the flattest of these accesses and its corresponding mating or faying surface, however, gaps may be present which reduce the efficiency of the shielding by containing openings through which radiant energy may leak or otherwise pass into or out of the device. Moreover, such gaps represent discontinuities in the surface and ground conductivity of the housing or other shielding, and may even generate a secondary source of EMI radiation by functioning as a form of slot antenna. In this regard, bulk or surface currents induced within the housing develop voltage gradients across any interface gaps in the shielding, which gaps thereby function as antennas which radiate EMI noise. In general, the amplitude of the noise is proportional to the gap length, with the width of the gap having a less appreciable effect.
For filling gaps within mating surfaces of housings and other EMI shielding structures, gaskets and other seals have been proposed both for maintaining electrical continuity across the structure, and for excluding from the interior of the device such contaminates as moisture and dust. Such seals are bonded or mechanically attached to, or press-fit into, one of the mating surfaces, and functions to close any interface gaps to establish a continuous conductive path there across by conforming under an applied pressure to irregularities between the surfaces. Accordingly, seals intended for EMI shielding applications are specified to be of a construction which not only provides electrical surface conductivity even while under compression, but which also has a resiliency allowing the seals to conform to the size of the gap. The seals additionally must be wear resistant, economical to manufacture, and capable of withstanding repeated compression and relaxation cycles.
Conductive materials for the filler, sheathing, or coating include metal or metal-plated particles, fabrics, meshes, and fibers. Preferred metals include copper, nickel, silver, aluminum, tin, or an alloy such as Monel, with preferred fibers and fabrics including natural or synthetic fibers such as cotton, wool, silk, cellulose, polyester, polyamide, nylon, and polyimide. Alternatively, other conductive particles and fibers such as carbon, graphite, or a conductive polymer material may be substituted.
These shielding devices are available in a wide range of sizes and can be supplied on continuous rolls or cut to length. Generally, these shielding devices are attached to the housing by rivets, welds, screws, etc., preferably, a continuous roll-formed strip metal clip, to which a shielding device has been attached, is used.
U.S. Pat. No. 5,008,485, issued to Kitagawa, discloses a conductive EMI shield including an inner seal member formed of an elastic, nonconductive material such as rubber or the like, and an outer conductive layer coated over the sealing member. Portions of the conductive layer extend beyond the seal member to directly contact the edges of a housing to which the sealing member is attached. The conductive layer is formed of a conductive compound comprising a resinous material which is filled with carbon black, a metallic powder, or the like to render it electrically conductive.
U.S. Pat. No. 5,028,739, issued to Keyser et al., discloses an EMI shielding gasket including a resilient, elastomeric core enveloped within a fine, open format knit or braided wire mesh. An adhesive strip is disposed lengthwise along a surface of the gasket allowing the gasket to be removably fastened directly to an enclosure.
U.S. Pat. No. 5,105,056, issued to Hoge, Jr., et al., discloses an EMI shielding gasket formed from a conductive sheathing which is wrapped circumferentially around a compressible core. Where the sheathing overlaps itself, a longitudinal seam is defined to which an adhesive is applied for bonding the gasket to a panel of an enclosure or the like. Preferably, the adhesive is applied discontinuously in a repetitive pattern of non-overlapping lines extending laterally across the seam.
U.S. Pat. No. 5,202,536, issued to Buonanno, discloses an EMI seal having an elongated resilient core which is covered with a partial conductive sheath. A conductive portion of the sheath, preferably a metallized fabric or the like in a resin binder, is provided to extend partially around the core to define ends which are non-overlapping. A second, nonconductive sheath portion is attached to the core element to extend between the ends of the conductive sheath portion. A contact adhesive may be used to hold the seal in place.
U.S. Pat. No. 6,121,545, issued to Peng, et. al., discloses a gasket providing a low closure force, particularly adapted for use in smaller electronic enclosure packages. The disclosed gasket has been designed to form a periodic “interrupted” pattern of alternating local maxima and minima heights. Gaskets of such type may be formed by molding, or using the form in place (FIP) process, and have a crenelated, i.e., notched, serrated, or a sinusoidal “waveform” profile, or as a series of discrete beads. In general, for a specified joint configuration, a gasket having such an “interrupted” profile or pattern would be expected to exhibit a greater deflection under a given compressive load than a continuous profile.
Another method of achieving a lower closure force in a spacer gasket design is described in commonly-assigned U.S. Pat. No. 6,121,545, issued to Peng et. al. This method involves configuring the gasket as having a moment arm portion which is angularly deflectable in an inward or outward direction relative to the frame responsive to a compressively-applied load. As a result of the described bending mode response, such gasket is seen to exhibit a force deflection as compared to gasket profiles operating in a conventional compression mode.
A typical small enclosure application generally requires a low impedance, low profile connection which is deflectable under relatively low closure force loads, e.g., about 1.0-8.0 lbs per inch (0.2-1.5 kg per cm) of gasket length. The deflection ensures that the gasket sufficiently conforms to the mating housing or board surfaces to develop an electrically conductive pathway therebetween. It has been observed that for certain applications, however, the closure or other deflection force required for certain conventional profiles may be higher than can be accommodated by the particular housing or board assembly design.
While the aforementioned and other known gaskets perform reasonably well, these gaskets are relatively costly to assemble in a cabinet. Moreover, the tightly knit wire mesh necessitates that a high closure force is required to seal the door or panel, and the combination of the tightly knit mesh and the required metal clip makes the gasket heavy, which is detrimental in applications where weight is a critical factor such as in the aerospace industry.
As the size of handheld electronic devices, such as cellular phone handsets, has continued to shrink, further improvements in the design of gasket profiles would be well-received by the electronics industry. Specifically, it is desirable to provide a low closure force gasket profile for use in smaller electronics enclosures which are increasingly becoming the industry standard.
Therefore, there is a need for a shielding gasket having a resilient core with a particular structure which is inexpensive and lightweight and allows a low closure force with an enclosing surface. The shielding gasket should also provide superior compression-deflection properties which are highly desirable in complex enclosures. Furthermore, the shielding gasket should be capable of being utilized in various bending curvatures of the enclosing surface, and be available in various shapes and profiles, such as asymmetrical or flat rectangular, C shape, V shape, D shape, P shape, etc., to accommodate new designs.
It is accordingly an objective of the present invention to provide a strip gasket which has a hollow cross section providing a lightweight structure and flexibility in fixing to an enclosing surface.
It is another objective of the present invention to provide a strip gasket which allows an enclosing surface to be sealed with a low closure force.
A further objective of the present invention is to provide a strip gasket which allows a lower electrical resistance.